Processing rare earth sulphate solutions

ABSTRACT

A method of processing a purified rare earth sulphate solution, the method including the steps of: contacting the purified rare earth sulphate solution with sodium hydroxide to precipitate rare earths as rare earth hydroxide, including the addition of an oxidant to oxidise cerium contained in the rare earth hydroxide precipitate; and selectively leaching the rare earth hydroxide precipitate with hydrochloric acid to form a rare earth chloride solution and a residue.

This application is the U.S. National Phase under 35 U.S.C. § 371 ofInternational Application PCT/AU2019/050403, filed May 2, 2019,designating the U.S., and published in English as WO 2019/210367 on Nov.7, 2019 which claims priority to Australian Patent Application No.2018901510, filed May 3, 2018, the entire content of which isincorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to the recovery of rare earthelements from rare earth ores or concentrates containing those elements.More particularly, the invention relates to the recovery of separatecerium and cerium free rare earth products from a rare earth sulphatesolution, such as a purified rare earth sulphate solution.

A purified rare earth sulphate solution may for example be one formedfrom a crude rare earth sulphate derived from the sulphation (acid bake)and subsequent water leach of a monazite ore, concentrate, or associatedpre-leach residue, the purified solution may for example be derived fromsuch a crude solution by way of the formation of a rare earth sulphateprecipitate followed by a water leach of the precipitate.

BACKGROUND OF THE INVENTION

Rare earth elements include all the lanthanide elements (lanthanum,cerium, praseodymium, neodymium, promethium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium,and lutetium), as well as the rare metals scandium and yttrium. For easeof discussion and because of their abundance and similar properties, theTotal Rare Earth Elements (TREE) are often divided into three groups;the Light Rare Earths (LRE) which are lanthanum, cerium, praseodymium,and neodymium; the Middle Rare Earths (MRE) which are samarium, europiumand gadolinium (promethium does not exist as a stable element); and theHeavy Rare Earths (HRE) which are terbium, dysprosium, holmium, erbium,thulium, ytterbium, and lutetium. Yttrium and Scandium are often addedto this list although they are not strictly heavy rare earth elements.

During the past twenty years there has been an explosion in demand formany items that require rare earth elements, which now include manyitems that people use every day, such as computer memory, DVD's,rechargeable batteries, mobile phones, catalytic converters, smallelectric motors, magnets, and fluorescent lighting.

Rare earth elements also play an essential role in electricitygeneration from wind power, new generation electric vehicles, andmilitary applications. In this respect, the military uses includenight-vision goggles, precision-guided weapons, communicationsequipment, GPS equipment and batteries.

The increase in use of rare earth elements in new technology devices haslead to an increase in demand, and a need for diversification in asupply chain that has been dominated by China since the early 1990s.Indeed, the development of non-Chinese resources for the mining andprocessing of rare earths has expanded in recent years, particularlysince China announced in 2010 that it will severely restrict its exportof rare earth elements to ensure supply for domestic manufacturing.

Rare earth containing minerals occurring in nature that are sufficientlyrich in rare earth elements to be of current or potential futurecommercial interest include fluoro-carbonates (such as bastnäsite),fluorides, oxides, phosphates (such as monazite, xenotime, and apatite),and silicates (such as ionic clays and allanite). The world's resourcesare contained primarily in bastnäsite and monazite. Bastnäsite depositsin China and the United States constitute the largest percentage of theworld's known rare-earth economic resources, while monazite deposits inAustralia, Brazil, China, India, Malaysia, South Africa, Sri Lanka,Thailand, and the United States constitute the second largest segment.

Conventional methods for the extraction of rare earth elements fromtheir ores or concentrates are described in the book ““ExtractiveMetallurgy of Rare Earths” by C. K. Gupta and N. Krishnamurthy, CRCPress, 2005.

As outlined in Gupta and Krishnamurthy (2005), rare earth elements havetypically been extracted from monazite ores and concentrates byprocesses of sulphation. In sulphation (also called “acid baking”), theore or concentrate is mixed with concentrated sulphuric acid and bakedat elevated temperatures (such as from 200 to 500° C.) to break down themineral matrix and convert the rare earth elements into sulphate saltsthat can then be brought into solution by dissolution in a water leachof the baked solids. Once the rare earth elements are in solution, beingimpure rare earth sulphate solutions that will herein be referred to as“crude rare earth sulphate solutions”, the rare earth elements aretypically recovered by a number of different prior art techniques.

Gupta and Krishnamurthy (2005) also describe processes for theextraction of rare earth elements from bastnäsite ores and concentrates(bastnäsite being a rare earth fluorocarbonate). In China, bastnäsiteconcentrates are processed by heating with concentrated sulphuric acidto 500° C. in a rotary kiln. The residue is then treated with water todissolve the soluble rare earth sulphates, again forming a crude rareearth sulphate solution.

One prior art process strategy that has been adopted to recover the rareearth elements contained in such crude rare earth sulphate solutions isto neutralise acid and precipitate problematic impurities, therebyforming “purified” rare earth sulphate solutions, followed by therecovery of the rare earth elements using one of several alternativeprocesses such as solvent extraction, rare earth carbonateprecipitation, rare earth oxalate precipitation, rare earth hydroxideprecipitation, or ion exchange. Another strategy is to directly recoverrare earth elements from crude rare earth sulphate solutions, for whichone process option is available in the prior art, namely double sulphateprecipitation.

Many of these process strategies can produce a rare earth hydroxideeither directly, as in hydroxide precipitation, or indirectly viacaustic conversion of intermediates, such as of double sulphate,carbonate or oxalate. The cerium contained in the resultant rare earthhydroxides can be oxidised with a strong oxidising agent, or by dryingin the presence of oxygen at 120 to 150° C. The mixed rare earthhydroxides may then be selectively leached in hydrochloric acid to pH 3to 4 to produce a rare earth chloride solution and a residue. Theproblem with existing processes, as described by Morals et al in“Recovery of Cerium by Oxidation/Hydrolysis With KMNO ₄-NA ₂ CO ₃”,Hydrometallurgy 2003—Fifth International Conference—Volume 2:Electrometallurgy and Environmental Hydrometallurgy, is that theselective dissolution is very slow and that about 1% of the cerium isleached into the rare earth chloride solution.

It is an aim of the present invention to provide a effective andefficient process for the selective recovery of rare earth elements frompurified rare earth sulphate solutions, and thus ultimately a moreeconomic overall process for producing separated rare earth productsfrom rare earth containing ore.

The above discussion of background is included to explain the context ofthe present invention. It is not to be taken as an admission that any ofthe material referred to was published, known, or part of the commongeneral knowledge (in any country) at the priority date of any one ofthe claims.

SUMMARY OF THE INVENTION

As mentioned above, in general terms in rare earth processing,sulphation is where an ore or concentrate is mixed with concentratedsulphuric acid and baked at elevated temperatures to break down themineral matrix and convert the rare earth elements into sulphate saltsthat can then be brought into solution by dissolution in a water leachof the baked solids. Once the rare earth elements are in solution, beingthe so-called “crude” rare earth sulphate solutions, the rare earthelements then require recovery.

For example, in the applicant's co-pending International patentapplication lodged on the same day as this application, there isdescribed a method for the precipitation of rare earth sulphate, whichincludes subjecting a crude rare earth sulphate solution toprecipitation in the presence of a water soluble, volatile, organiccompound to produce a rare earth sulphate precipitate and an acidicsupernatant. The rare earth sulphate precipitate is then washed anddried, and subsequently leached in water to dissolve soluble rare earthsulphate and form a leach solution rich in rare earth sulphate and aleach residue containing impurities in the form of insoluble phosphates.The rare earth sulphate leach solution is then subjected to theprecipitation of impurities to form a “purified rare earth sulphatesolution” and a purification residue. It is such a “purified” rare earthsulphate solution that the present invention applies to. The presentinvention could equally be applied to a purified rare earth sulphateprepared by the neutralisation of crude rare earth sulphate such asdescribed in the background.

The present invention thus provides a method of processing a purifiedrare earth sulphate solution, the method including the steps of:

-   -   a) contacting the purified rare earth sulphate solution with        sodium hydroxide to precipitate rare earths as rare earth        hydroxide, including the addition of an oxidant to oxidise        cerium contained in the rare earth hydroxide precipitate; and    -   b) selectively leaching the rare earth hydroxide precipitate        with hydrochloric acid to form a rare earth chloride solution        and a residue.

In one form, in the preparation of a purified rare earth sulphatesolution, the washing of a rare earth sulphate precipitate formed from acrude rare earth sulphate solution is conducted with a water-soluble,volatile, organic compound, such as methanol, with the sulphateprecipitate then being dried (ideally with volatilised organic recoveredand reused) prior to a water leach of the sulphate precipitate. Such awater leach preferably occurs at about 40° C. for up to 180 minutes todissolve the soluble rare earth sulphate, while leaving most impuritiesbehind as insoluble phosphates (such as thorium) which may be recycledor disposed. Trace quantities of soluble impurities will also bedissolved in such a process.

Then, following solid-liquid separation, the leach residue may bedisposed while the resultant leach solution rich in rare earth sulphateideally subsequently undergoes a rare earth sulphate purification stageto form the purified rare earth sulphate solution relevant to thepresent invention and a purification residue.

In relation to this purification stage, magnesia is preferably added tothe rare earth sulphate leach solution to purify that solution byprecipitation of impurities, leaving the desired purified rare earthsulphate solution and a purification residue. The purification isideally operated to maximise the precipitation of impurities whileminimising the co-precipitation of rare earth elements. As such,magnesia is preferably dosed over multiple tanks at a temperature up toabout 55° C., with a 30 to 120 minute residence time in each tank, to apH 4.5 to 6.0 end point target. Then, following solid liquid separation,the purification precipitate may be recycled or disposed, and thepurified rare earth sulphate solution recovered for further processing.

Returning to a description of the processing method of the presentinvention, sodium hydroxide is added to the purified rare earth sulphatesolution in order to precipitate the rare earths as rare earthhydroxide, with the addition of the oxidant assisting to oxidise ceriumcontained in the precipitate. The precipitation step a) is thus ideallyoperated to maximise the precipitation of rare earth elements andmaximise the conversion of cerium (III) to cerium (IV), while minimisingthe stoichiometric excess of reagent dosing, and minimise theconcentration of sulphate in the rare earth hydroxide precipitate.

The precipitation step a) preferably occurs in a two-stagecounter-current process, including a precipitation stage and refiningstage, wherein the rare earth sulphate solution feeds into theprecipitation stage with spent solution from the refining stage, toprecipitate rare earth hydroxide containing sulphate. The sulphatecontaining rare earth hydroxide may then be converted to clean rareearth hydroxide in the refining stage with the addition of fresh sodiumhydroxide.

In a preferred form, both the precipitation and the refining stageoperate at a temperature of 40 to 80° C., and for a time of about 30 to60 minutes, with the stoichiometry of sodium hydroxide addition being inthe range of 100 to 110%, and with 100 to 130% of stoichiometry dosingof oxidant.

Preferably, the oxidant is hydrogen peroxide and/or sodium hypochlorite,which is added to the precipitation and/or refining stages followingsodium hydroxide addition. For the case where hydrogen peroxide is theoxidant, it is preferred that the operating temperature be in the range50 to 60° C. at the point(s) of addition.

In the leaching step b), the selective leaching of rare earth hydroxideprecipitate is preferably conducted in two stages, each stage havingmultiple tanks and each stage using hydrochloric acid diluted to around10% w/w using leach solution from the first leach stage. Leach solutionfrom the second leach stage is preferably used to re-pulp, leach rareearth hydroxide precipitate, and to precipitate cerium (IV) that hasbeen dissolved in the second stage, prior to the first leach stage.

The selective leaching ideally occurs at a temperature of 60 to 80° C.,with the first leach stage being operated to maximise rare earthdissolution while minimising cerium (IV) dissolution, which is achievedwith an endpoint pH of about pH 3 to 4, and the second leach stage beingoperated to minimise the concentration of non-cerium rare earth elementsin the residue. In this respect, it is preferred for the rare earthchloride solution to contain negligible cerium, and for the residue toconsist primarily of cerium (IV) hydroxide.

The cerium (IV) hydroxide residue may be packaged as a crude ceriumproduct or further processed to produce a higher purity cerium product.Additionally, barium chloride may subsequently be added to the rareearth chloride solution, with sulphuric acid when sulphate levels arelow, to remove radium via co-precipitation with barium sulphate to forma purified rare earth chloride solution. The purified rare earthchloride solution may then be concentrated by evaporation.

Dilute and/or concentrated purified rare earth chloride may then beseparated into two or more different rare earth products by solventextraction with each rare earth product containing one or moreindividual rare earth elements.

The method of the invention may also include some additional stepsrelated to the production of a suitable crude rare earth sulphatesolution from a rare earth rich calcium phosphate concentrate, mostlikely where the rare earth rich calcium phosphate concentrate is aproduct of the beneficiation of a rare earth containing calciumphosphate rich ore by whole-of-ore flotation. In a preferred form, theore contains apatite-hosted monazite, and the concentrate contains ahigher grade of apatite-hosted monazite relative to the ore.

In general terms, these pre-processing steps preferably include thepre-leach of a rare earth rich calcium phosphate concentrate withphosphoric acid to form a pre-leach residue enriched in rare earths, themixing of the pre-leach residue with sulphuric acid with subsequentheating of the mixture to convert the rare earths in the pre-leachresidue to water-soluble rare earth sulphates, and finally a water leachof the heated mixture to place the rare earth sulphates in solution andthereby form the crude rare earth sulphate solution suitable for theprecipitation step described above.

The crude rare earth sulphate solution may then be subjected toprecipitation in the presence of a water soluble, volatile, organiccompound to produce a rare earth sulphate precipitate and an acidicsupernatant.

Ideally, the rare earth sulphate precipitation step is operated tomaximise the extent of rare earth precipitation from solution whileminimising rare earth precipitation in the form of phosphates andfluorides, and minimising the co-precipitation of impurities. The rareearth sulphate precipitate is expected to contain >35% TREE, >12% S, <2%P, <0.3% Al, and <0.5% Fe, while all organic added to the precipitation,less that volatilised, will be contained in the acid supernatant.Indeed, virtually all the water, sulphuric acid, phosphoric acid, Mg,Al, Fe, and U contained in the crude rare earth sulphate solution shouldbe contained in the acidic supernatant.

While the rare earth sulphate precipitate subsequently undergoes furtherprocess steps to form the purified rare earth sulphate solution of thepresent invention, the acidic supernatant produced may also undergofurther process steps aimed at the recovery of the organic compound andthe regeneration of phosphoric acid. This may include recovering theorganic compound from the acidic supernatant by distillation, resultingin the formation of recovered organic compound and a dilute mixed acidsolution, with the recovered organic compound preferably being recycledfor use in the precipitation step, and the dilute mixed acid solutionbeing passed on directly and/or following additional concentration byevaporation for use in a phosphoric acid regeneration stage that willalso be further described below.

Following the phosphoric acid pre-leach of the pre-processing stagesdescribed above, heat may be applied to the phosphoric acid pre-leachsolution, to precipitate out of solution any minor amounts of rareearths as rare earth phosphates, leaving a recovery solution. It isthese rare earth phosphates that may then be returned to the sulphationand acid bake steps described above.

The recovery solution may then be dosed with sulphuric acid and/or thedilute and/or concentrated mixed acid solution of the methanol-strippingstage to convert mono calcium phosphate to phosphoric acid and form acalcium sulphate precipitate which may be disposed. The target is abalance between maximising the reactivity of phosphoric acid andminimising the amount of calcium sulphate precipitate forming in thepre-leach mentioned above.

The phosphoric acid formed from the recovery solution may then be usedas required by the pre-leach in the pre-processing steps describedabove, and the surplus phosphoric acid may be bled out of the system andforwarded on for phosphoric acid purification.

In such phosphoric acid purification, impurities (primarily uranium andthorium) in the dilute phosphoric acid bleed may be removed by ionexchange, solvent extraction or any other commercial process along withsome rare earth elements. In addition, the concentration of calcium andsulphur in the dilute acid may be adjusted through the addition ofsulphuric acid as required to meet consumer specifications. The purifiedand dilute phosphoric acid may then be concentrated up to generate aphosphoric acid by-product, which may be stored prior to shipment tocustomers.

DESCRIPTION OF PREFERRED EMBODIMENT

FIG. 1 is a schematic representation of a flow diagram for theprecipitation of rare earth sulphates, which also illustrates apreferred pre-processing stage for the rare earth rich calciumphosphates, and a preferred associated phosphoric acid regenerationstage; and

FIG. 2 is a schematic representation of a flow diagram for a preferredembodiment of a method of processing a purified rare earth sulphatesolution in accordance with the present invention.

DETAILED DESCRIPTION OF THE FLOW DIAGRAM

Before providing a more detailed description of a preferred embodimentof the present invention with reference to experimental data, it will beuseful to provide some explanation of the flow diagram of FIGS. 1 and 2.

FIG. 1 shows the precipitation of rare earth sulphates (section A),together with the pre-processing of rare earth rich calcium phosphates(section B) and an associated phosphoric acid regeneration stage(section C). In FIG. 2 , there is shown a series of rare earthprocessing stages (section D). It will be appreciated that the focus ofthe present invention is on the rare earth processing stages in sectionD.

Referring firstly to FIG. 1 , and in process flow order starting withsection B, various steps can be seen relating to the production of asuitable crude rare earth sulphate solution 10 from a rare earth richcalcium phosphate concentrate 12, being a product of the beneficiationof a rare earth containing calcium phosphate rich ore by whole-of-oreflotation (not shown).

These pre-processing steps include the pre-leach of the concentrate withphosphoric acid in a multi stage counter current configuration 14,16 toremove calcium phosphate and to form a pre-leach residue 18 enriched inrare earths and a pre-leach solution 19 containing monocalciumphosphate, impurities and minor amounts of rare earths. In thisembodiment, the leach 14,16 is operated at a low temperature (typically30 to 45° C.), with residence times kept to between 30 to 90 minutes foreach stage, and with the overall feed acid to feed concentrate massratio kept between 2 and 12 grams of P in the acid per gram of Ca in theconcentrate.

The pre-leach residue 18 is then combined and cracked using concentratedsulphuric acid in a sulphation stage, being mixed 20 with sulphuric acid22 either at or below an acid bake temperature for a time of up to about30 minutes to ensure that the resulting mixture is fully homogenisedprior to the next steps. Subsequent heating 24 of the mixture convertsthe rare earths in the pre-leach residue 18 to water-soluble rare earthsulphates 26.

The water-soluble rare earth sulphates 26 are then cooled to atemperature less than about 50° C. over a time period of up to about 300minutes to remove as much heat out of the sulphated material 26discharging the acid bake 24 as is practical prior to its use in asubsequent water leach 28.

The cooled water-soluble rare earth sulphates 27 are then subjected to awater leach 28 to place in solution the rare earth sulphate, phosphoricacid and any remaining sulphuric acid, and thus to form the “crude” rareearth sulphate solution 10 mentioned above and a water leach residue 30containing insoluble gangue material for disposal.

Turning now to the next section, section A, being the precipitation ofrare earth sulphates, FIG. 1 shows the sulphate precipitation stage 32where the crude rare earth sulphate solution 10 is subjected toprecipitation in the presence of a water soluble, volatile, organiccompound (such as methanol) 34 to produce a rare earth sulphateprecipitate 36 and an acidic supernatant 38. The sulphate precipitation32 ideally occurs at a temperature in the range of 60 to 65° C. with aresidence time in the range of 20 to 40 minutes.

While the rare earth sulphate precipitate 36 subsequently undergoesfurther process steps in accordance with the present invention, whichwill be described below in relation to FIG. 2 , the acidic supernatant38 produced in the sulphate precipitation step 32 also undergoes furtherprocess steps (section C) aimed at the recovery of the organic compoundand the regeneration of phosphoric acid.

Section C of FIG. 1 thus shows recovering the organic compound from theacidic supernatant 38 by distillation 40, resulting in the formation ofrecovered organic compound 34 and a dilute mixed acid solution 42, withthe recovered organic compound 34 being recycled for use in the sulphateprecipitation step 32 as the methanol 34, and the dilute mixed acidsolution 42 being subjected to additional concentration 44 byevaporation to form a concentrated mixed acid solution 45 for use in aphosphoric acid regeneration stage 46.

Following the phosphoric acid pre-leach 14,16 mentioned above, heat isapplied 48 to the phosphoric acid pre-leach solution 19 to precipitateout of solution any minor amounts of rare earths as rare earthphosphates 50, leaving a recovery solution 52. As can be seen, the rareearth phosphates 50 are then returned to the acid mix 20 and acid bake24 steps described above. The heating 48 of the phosphoric acidpre-leach solution 19 occurs in several stages of increasingtemperatures, the temperatures all being in the range of 60° C. to 110°C., with stage residence times of between 60 and 180 minutes.

The recovery solution 52 is then dosed with sulphuric acid 54 in theacid regeneration stage 46 and, in this embodiment, also with theconcentrated mixed acid solution 45, to convert mono calcium phosphateto recoverable phosphoric acid 56 and form a calcium sulphateprecipitate 57 which may be disposed. This acid addition 46 is conductedin stages at a temperature of about 40° C. and a residence time ofbetween 30 and 60 minutes per stage. Part of the phosphoric acid formedfrom the recovery solution 58 is also used by the pre-leach 14,16, whilesurplus phosphoric acid 59 is bled out of the system.

Turning now to FIG. 2 , and the final, inventive, section (section D) ofthe flow diagram, which shows the rare earth processing stages, thissubsequent processing includes washing and drying the rare earthsulphate precipitate (actually shown in FIG. 1 as stage 60), andsubsequently leaching 62 in water the washed and dried rare earthsulphate precipitate 36 to dissolve soluble rare earth sulphate and forma leach solution 64 rich in rare earth sulphate and a leach residue 66containing impurities in the form of insoluble phosphates.

Then, impurities are precipitated 68, with the addition of magnesia 63,from the rare earth sulphate leach solution 64 to form a “purified” rareearth sulphate solution 70 and a purification residue 72, followed bythe precipitation 74 of the rare earths in the purified rare earthsulphate solution 70 as rare earth hydroxide precipitate 76.

Sodium hydroxide 77 is added to the purified rare earth sulphatesolution 70 to precipitate the rare earths as rare earth hydroxide 76,with the addition of hydrogen peroxide 78 to oxidise cerium contained inthe precipitate. The production of rare earth hydroxide 76 occurs in atwo-stage counter current process, with purified rare earth sulphatesolution 70 feeding into the precipitation stage (stage one) 74 toprecipitate a crude rare earth hydroxide 75 containing some sulphateusing spent solution 71 from the refining stage (stage two) 73, with theconversion of rare earth sulphate compounds to rare earth hydroxidecompounds occurring with the addition of fresh sodium hydroxide 77 inrare earth hydroxide refining (stage two) 73.

The rare earth hydroxide precipitate 76 then undergoes selectiveleaching 79,80,82 with hydrochloric acid to form the rare earth chloridesolution 86 and the residue 88 with the rare earth solution 86containing negligible cerium, and the residue 88 consisting primarily ofcerium (IV) hydroxide.

The selective leaching 79,80,82 of the rare earth hydroxide precipitate76 is conducted in a multi stage configuration, with hydrochloric aciddiluted to 10% w/w using polished stage one 80 leach solution prior toits addition to stage one 80 and stage two 82 leach tanks, over multipletanks in each stage, and stage two 82 solution is used to re-pulp andleach rare earth hydroxide cake prior to stage one 80 leach.

The cerium (IV) hydroxide residue 88 is then packaged as a crude ceriumproduct, while the rare earth chloride solution 86 is dosed with bariumchloride 90 to remove radium via co-precipitation with barium sulphateand form a purified rare earth chloride solution 92. The purified rareearth chloride solution 92 is then concentrated by evaporation 94.

DESCRIPTION OF EXPERIMENTAL DATA

Attention will now be directed to a description of experimental dataacross the entire flowsheet, developed to illustrate a preferredembodiment of the present invention.

Rare Earth Sulphate Precipitation

Rare earth sulphate precipitation tests were conducted by contacting ameasured quantity of pre-heated water leach solution with a measuredquantity of ambient temperature methanol in a suitable well agitatedbaffled vessel fitted with reflux condenser to minimise evaporativeloss. The resulting mixture was maintained at a setpoint temperature fora specified duration, then vacuum filtered. The filter cake was thenwashed thoroughly with methanol to remove entrained solution prior todrying.

The Influence of Temperature (Tests 1 and 2)

Two rare earth sulphate precipitation tests were conducted to evaluatethe influence of reaction temperature on performance. One test wasconducted at 60 to 65° C. (Test 1) while the other was conducted at 40to 45° C. Both tests were conducted on the same water leach solution(Table 1) and were contacted with 1 gram of methanol per gram of waterleach solution for 30 minutes. The results are summarised in Tables 2 to4. The results indicate that operating at lower temperatures results inreduced Al, P, Fe, Th, and U co-precipitation with rare earth sulphate.

TABLE 1 Feed solution composition LRE MRE HRE Y TRE Al P S Ca Fe Th UTest g/L mg/L mg/L mg/L g/L g/L g/L g/L mg/L g/L g/L mg/L 1 & 2 15.0 71090 130 15.9 2.69 33.2 76.5 339 3.32 1.59 156

TABLE 2 Final solution composition LRE MRE HRE Y TRE Al P S Ca Fe Th UTest mg/L mg/L mg/L mg/L mg/L g/L g/L g/L mg/L g/L g/L mg/L 1 427 92 2864 610 1.46 19.2 39.6 73 1.80 660 77 2 474 95 28 63 659 1.39 18.6 39.656 1.77 710 77

TABLE 3 Rare earth sulphate precipitate composition (% w/w or g/t) LREMRE HRE Y TRE Al P S Ca Fe Th U Test % % g/t g/t % g/t % % % g/t % g/t 140.3 1.21 611 757 41.6 476 0.23 14.1 0.41 629 0.76 1.6 2 40.7 1.17 514637 42.0 424 0.18 14.1 0.46 559 0.47 1.2

TABLE 4 Precipitation extent (%) Test LRE MRE HRE Y TRE Al P S Ca Fe ThU 1 94.8 71.7 29.9 18.7 92.9 0.6 0.2 6.4 52.3 0.7 18.1 0.0 2 94.0 69.125.1 15.6 92.1 0.6 0.2 6.1 59.7 0.6 10.7 0.0The Influence of Contact Ratio and Residence Time (Tests 3 to 9)

Two sets of rare earth sulphate precipitation tests were conducted toevaluate the influence of organic to aqueous contact ratio and residencetime on performance. Four contact ratios (0.25, 0.5, 0.75, and 1) weretested in the first set of tests (Tests 3 to 6 respectively). Subsampleswere collected at 30, 60, and 120 minutes (subsamples A, B, and C) ineach of the tests 3 to 6. The second set of tests (Tests 7 to 9)evaluated shorter residence times (10, 20, and 30 minutes respectively)with a contact ratio of 1 using a different feed solution to the firstset (see Table 5). All tests (Tests 3 to 9) used methanol as the organicphase, and were conducted with a 60 to 65° C. temperature target whileexperienced temperatures ranging between 55 to 70° C. The results aresummarised in Tables 6 to 8.

The results indicate that operating at lower organic to aqueous contactratios reduces the rare earth element precipitation extent. The resultsindicate that rare earth sulphate precipitation is effectively completewithin the shortest residence time tested (10 minutes in Test 7), whilethe precipitation of impurities such as thorium are effectively completeby 20 minutes, with negligible additional precipitation observed withextended contact durations.

TABLE 5 Feed solution composition LRE MRE HRE Y TRE Al P S Ca Fe Th UTest g/L mg/L mg/L mg/L g/L g/L g/L g/L mg/L g/L g/L mg/L 3-6 29.2 1048162 241 30.7 3.3 14.4 53.4 950 3.0 2.33 235 7-9 24.6 890 115 205 25.84.0 14.1 40.4 1220 2.4 1.90 168

TABLE 6 Final solution composition LRE MRE HRE Y TRE Al P S Ca Fe Th UTest mg/L mg/L mg/L mg/L mg/L g/L g/L g/L mg/L g/L g/L mg/L 3A 274 89 3986 488 1.5 6.01 19.3 130 1.3 0.91 106 3B 291 90 40 87 508 1.5 6.04 19.1140 1.4 0.93 107 3C 264 90 40 91 485 1.6 6.18 18.7 140 1.5 0.97 113 4A674 106 43 96 919 1.7 7.08 23.3 170 1.6 1.01 121 4B 782 107 45 98 10321.7 7.23 24.2 180 1.6 1.05 126 4C 588 97 42 94 821 1.7 6.79 22.0 170 1.60.96 121 5A 1721 211 65 133 2130 2.1 8.23 25.8 250 1.9 1.29 147 5B 1839222 67 141 2269 2.2 8.78 29.8 260 1.9 1.38 154 5C 2544 248 73 151 30162.3 9.43 32.2 300 2.1 1.50 162 6A 5721 457 100 176 6453 2.6 10.4 37.0520 2.4 1.80 184 6B 5287 448 100 186 6021 2.7 10.4 35.2 530 2.4 1.86 1866C 7265 498 106 193 8062 2.8 11.0 36.4 570 2.6 1.98 191 7 389 56 24 58527 1.7 6.05 14.2 80 1.1 0.39 75 8 443 58 23 57 580 1.7 6.20 16.3 <501.0 0.28 75 9 394 52 22 54 523 1.8 6.29 16.8 <50 1.0 0.27 78

TABLE 7 Rare earth sulphate precipitate composition (% w/w or g/t) LREMRE HRE Y TRE Al P S Ca Fe Th U Test % % g/t g/t % g/t % % % g/t % g/t3A 40.7 1.19 695 1030 42.1 106 0.37 14.8 0.74 769 1.06 5.1 3B 40.5 1.18682 1000 41.8 106 0.37 14.9 0.77 769 1.01 4.7 3C 40.6 1.17 698 1020 41.9106 0.38 14.9 0.80 769 1.06 5.5 4A 40.4 1.21 801 1260 41.9 212 0.41 14.90.77 839 1.16 7.5 4B 39.2 1.16 755 1140 40.5 212 0.39 14.7 0.76 839 1.086.0 4C 39.5 1.18 758 1140 40.9 159 0.41 14.8 0.80 839 1.16 5.4 5A 40.31.06 576 826 41.5 106 0.40 14.9 0.76 769 0.94 6.0 5B 39.5 1.09 567 80140.7 53 0.39 14.8 0.73 769 0.93 4.5 5C 39.8 1.08 570 800 41.0 53 0.4014.8 0.77 699 0.94 5.3 6A 41.0 0.90 369 473 42.0 >53 0.35 14.7 0.49 4900.62 2.0 6B 40.3 0.88 355 453 41.2 >53 0.34 14.7 0.47 560 0.60 2.1 6C40.8 0.87 355 461 41.7 >53 0.38 14.7 0.55 525 0.58 2.5 7 38.9 1.21 8811350 40.3 1641 1.21 14.1 1.52 2238 2.12 30 8 37.9 1.20 939 1440 39.32487 1.89 13.6 1.65 3567 2.65 53 9 38.1 1.26 997 1590 39.6 2540 1.8913.7 1.67 3357 2.74 52

TABLE 8 Precipitation extent (%) Test LRE MRE HRE Y TRE Al P S Ca Fe ThU 3A 97.8 80.3 35.3 26.8 96.3 0.2 1.9 18.9 63.6 1.8 26.2 0.1 3B 97.880.4 35.1 26.4 96.3 0.2 1.9 19.6 63.3 1.7 25.3 0.1 3C 98.1 81.4 36.627.2 96.7 0.2 2.0 21.0 65.7 1.7 26.9 0.2 4A 95.5 80.0 39.4 31.5 94.1 0.42.0 18.3 61.4 1.8 28.7 0.2 4B 94.7 79.6 37.8 29.5 93.4 0.4 1.9 18.0 60.21.9 27.0 0.2 4C 96.3 82.3 41.1 31.7 95.0 0.4 2.3 20.4 64.3 2.0 31.7 0.25A 90.3 66.7 26.1 19.8 88.6 0.2 1.9 18.7 54.6 1.6 22.4 0.2 5B 89.7 66.425.3 18.6 87.9 0.1 1.8 16.7 53.1 1.6 21.4 0.1 5C 87.0 65.0 25.0 18.585.3 0.1 1.8 16.5 52.2 1.4 21.1 0.1 6A 72.9 42.4 12.2 9.2 71.0 — 1.213.0 26.3 0.8 11.5 0.0 6B 76.7 46.0 13.4 9.5 74.8 — 1.4 15.3 27.8 1.012.2 0.0 6C 69.3 41.3 11.9 8.7 67.5 — 1.4 14.0 27.8 0.8 10.5 0.1 7 96.786.1 52.0 40.3 95.7 2.7 5.5 22.4 84.6 5.6 61.3 1.2 8 96.2 86.1 54.8 43.295.3 4.2 8.3 19.9 — 9.6 73.9 2.1 9 96.7 88.1 57.7 47.4 95.9 4.1 8.4 20.0— 9.3 75.5 2.0The Influence of Feed Composition (Tests 10 to 24)

Fifteen rare earth sulphate precipitation tests were conducted toevaluate the influence of variation in feed composition on precipitationperformance. Each test used a different crude rare earth sulphate feedsolution (Table 9), was contacted with methanol with a 1 to 1 w/wcontact ratio for 30 minutes, with a 60 to 65° C. temperature targetwhile experienced temperatures ranging between 55 to 70° C. The resultsare summarised in Tables 10 to 12, from which it can be seen that mosttests resulted in a relatively clean rare earth sulphate precipitatewith some variation in iron aluminium and phosphate co-precipitation.

To understand this impurity co-precipitation, the results have beencondensed down and sequenced according to the free sulphuric acidcontent of crude rare earth sulphate feed solution for each test (Table13). This free acid value is a calculated value for convenience and maynot reflect the actual speciation which takes place in the feedsolution. The free acid content was determined by summing up all thecations and anions measured and inferred from solution assay result withthe concentration of protons calculated to balance cations and anions.It was then assumed that all phosphate is present as phosphoric acidwith remaining protons assigned as sulphuric acid.

For Test 13 this resulted in a negative content of sulphuric acid whichsuggests that either the phosphate is not fully protonated or that assayuncertainty has biased cations over anions. Either way, Test 13 is lowon free acid, and a significant fraction of the precipitating rare earthelements have precipitated as phosphates. Overall the results suggestthat in order to minimise the precipitation of rare earth elements asphosphates, the from phosphate containing crude rare earth sulphatesolution should contain 5% w/w or more free H₂SO₄ as it has been definedhere.

TABLE 9 Feed solution composition LRE MRE HRE Y TRE Al P S Ca Fe Th UTest g/L mg/L mg/L mg/L g/L g/L g/L g/L mg/L g/L g/L mg/L 10 13.4 508 73122 14.1 0.9 6.25 24.8 1180 0.80 0.78 110 11 11.8 449 64 108 12.4 1.711.2 51.3 1043 1.28 0.69 97 12 20.7 625 79 155 21.6 7.6 31.3 50.4 8104.90 2.24 193 13 20.8 684 85 163 21.7 12.1 40.1 25.4 3320 3.30 1.17 16714 27.7 985 131 233 29.1 8.0 29.5 47.9 850 3.80 2.47 222 15 27.6 1073153 232 29.0 5.39 37.9 112 420 4.71 2.35 228 16 34.1 1424 167 288 35.93.83 26.6 99.7 818 3.41 3.48 307 17 32.8 1323 158 267 34.5 3.23 23.891.6 573 2.37 3.29 285 18 34.2 1534 170 332 36.3 2.65 24.8 106 575 3.963.73 338 19 22.5 802 116 204 23.6 1.58 31.4 95.0 720 0.59 2.24 197 2024.6 907 130 233 25.9 1.71 33.2 106 525 0.91 2.51 225 21 15.5 574 109224 16.4 2.21 39.4 129 306 1.40 2.86 265 22 9.62 345 85 186 10.2 2.8444.4 157 147 2.30 3.35 319 23 8.69 245 65 149 9.14 3.52 51.2 184 94 3.293.76 367 24 15.4 493 64 123 16.1 6.89 48.0 89.5 464 5.25 1.81 169

TABLE 10 Final solution composition LRE MRE HRE Y TRE Al P S Ca Fe Th UTest mg/L mg/L mg/L mg/L mg/L g/L g/L g/L mg/L g/L mg/L mg/L 10 189 2111 28 250 0.3 2.01 8.51 80 0.3 68 49.8 11 269 49 17 35 371 0.7 4.52 20.1230 0.5 353 43.5 12 590 52 17 45 704 3.5 12.4 16.5 100 1.9 617 83.4 13104 15 6 19 144 3.7 13.3 8.87 110 0.6 20 53.3 14 413 37 16 42 509 2.410.8 15.0 <50 1.5 414 80.6 15 280 87 36 67 468 2.5 17.5 45.3 125 2.17817 99.0 16 392 71 30 69 563 1.58 10.7 36.0 190 1.38 923 125 17 290 3522 55 401 1.29 9.97 34.5 100 0.97 816 130 18 393 82 39 83 597 1.03 11.144.1 120 1.63 1165 159 19 354 59 24 56 494 0.63 13.8 39.7 108 0.25 62981.6 20 320 68 27 62 478 0.72 15.0 45.6 87 0.39 692 86.6 21 366 68 28 65526 0.88 17.4 5.24 64 0.56 896 102 22 345 52 22 54 472 0.93 18.4 5.06 280.75 937 102 23 477 49 20 50 596 1.37 24.6 7.16 10 1.26 1275 134 24 44160 17 40 557 3.09 23.8 3.52 55 2.21 732 71.9

TABLE 11 Rare earth sulphate precipitate composition (% w/w or g/t) LREMRE HRE Y TRE Al P S Ca Fe Th U Test % % g/t g/t % % % % % % % g/t 1035.8 1.22 1172 1890 37.3 0.11 1.62 14.2 2.12 0.21 2.24 45 11 38.1 1.08670 932 39.4 0.005 0.29 15.3 1.44 0.05 0.79 2.6 12 38.8 1.01 706 119039.9 0.15 1.66 13.6 1.33 0.16 1.90 29 13 24.3 0.77 799 1410 25.3 3.3811.4 7.21 3.58 2.20 3.35 410 14 36.5 1.24 964 1510 38.0 0.12 1.73 12.40.97 0.17 2.42 19 15 40.0 1.18 712 1050 41.4 <0.005 0.22 14.7 0.32 0.060.61 1.4 16 40.1 1.23 910 1470 41.6 0.005 0.23 14.6 0.49 0.06 1.24 3.617 40.1 1.28 1225 2200 41.7 0.005 0.24 14.6 0.44 0.06 1.70 4.5 18 39.71.38 1056 1670 41.4 <0.005 0.21 14.5 0.40 0.07 1.53 5.4 19 40.1 1.22 8651360 41.6 <0.005 0.28 14.9 0.73 0.04 1.17 2.9 20 41.7 1.29 926 1430 43.2<0.005 0.25 14.8 0.41 0.04 1.15 2.2 21 41.8 1.17 979 1550 43.3 <0.0050.23 14.7 0.29 0.04 1.44 2.4 22 41.2 0.97 898 1520 42.4 <0.005 0.23 14.70.18 0.05 1.88 2.4 23 40.9 0.68 596 1040 41.7 <0.005 0.25 15.0 0.30 0.051.66 7.0 24 39.7 0.88 498 787 40.7 0.02 0.73 14.7 0.93 0.06 0.51 5.5

TABLE 12 Precipitation extent (%) Test LRE MRE HRE Y TRE Al P S Ca Fe ThU 10 96.3 88.8 60.3 48.2 95.4 4.9 10.0 18.8 78.6 8.8 82.0 1.2 11 94.673.3 32.4 24.7 93.0 0.1 0.8 8.7 43.7 1.2 21.7 0.1 12 93.3 80.5 46.8 36.192.4 0.9 2.8 15.0 73.9 1.8 39.7 0.7 13 98.8 94.6 81.3 72.1 98.4 23.822.7 21.8 91.8 55.7 98.3 20.9 14 95.8 89.5 60.4 48.0 95.1 1.3 4.0 17.7 —2.9 60.2 0.6 15 97.5 78.4 34.8 29.5 95.9 — 0.3 8.0 40.7 0.7 16.5 0.0 1696.5 82.3 44.9 36.4 95.2 0.1 0.6 9.8 41.1 1.1 26.5 0.1 17 97.6 91.6 62.254.0 96.8 0.1 0.7 11.1 56.6 1.7 38.0 0.1 18 96.5 82.3 43.1 35.7 95.0 —0.5 8.4 48.0 1.2 26.7 0.1 19 96.1 81.7 43.7 34.7 94.8 — 0.4 7.6 59.5 3.528.8 0.1 20 96.8 81.2 43.8 34.5 95.4 — 0.4 6.9 51.8 2.4 27.6 0.1 21 94.070.2 32.1 24.6 91.8 — 0.2 3.7 37.9 0.8 18.0 0.0 22 89.1 56.1 21.7 16.286.0 — 0.1 1.9 30.3 0.4 12.0 0.0 23 85.6 49.2 17.0 12.7 82.9 — 0.1 1.467.6 0.3 8.3 0.0 24 92.9 68.3 30.0 22.5 91.4 0.1 0.4 5.7 71.1 0.4 9.20.1

TABLE 13 Rare earth sulphate composition and precipitation extents as afunction of free acid Feed Free Acid Precipitate CompositionPrecipitation % w/w (% w/w) Extent (%) Test H₂SO₄ TREE P Al Fe TREE P AlFe 13 −2.5 25.3 11.4 3.38 2.20 98.4 22.7 23.8 55.7 10 4.3 37.3 1.62 0.110.21 95.4 10.0 4.9 8.8 14 4.5 38.0 1.73 0.12 0.17 95.1 4.0 1.3 2.9 125.7 39.9 1.66 0.15 0.16 92.4 2.8 0.9 1.8 11 9.9 39.4 0.29 0.005 0.0593.0 0.8 0.1 1.2 24 13.7 40.7 0.73 0.02 0.06 91.4 0.4 0.1 0.4 17 14.841.7 0.24 0.005 0.06 96.8 0.7 0.1 1.7 16 15.9 41.6 0.23 0.005 0.06 95.20.6 0.1 1.1 19 16.1 41.6 0.28 <0.005 0.04 94.8 0.4 — 3.5 18 16.9 41.40.21 <0.005 0.07 95.0 0.5 — 1.2 15 17.7 41.4 0.22 <0.005 0.06 95.9 0.3 —0.7 20 17.7 43.2 0.25 <0.005 0.04 95.4 0.4 — 2.4 21 21.3 43.3 0.23<0.005 0.04 91.8 0.2 — 0.8 22 25.2 42.4 0.23 <0.005 0.05 86.0 0.1 — 0.423 28.1 41.7 0.25 <0.005 0.05 82.9 0.1 — 0.3Phosphoric Acid Pre-Leach

Pre-leach tests were conducted by contacting a measured quantity oftemperature controlled phosphoric acid solution with a measured quantityof concentrate in a suitable well agitated baffled vessel. The resultingmixture was maintained at a setpoint temperature for a specifiedduration, then vacuum filtered. The filter cake was then washedthoroughly with DI water to remove entrained solution prior to drying.

The Influence of Concentrate Feed Variability

Five pre-leach tests were conducted to evaluate the influence ofvariability in the feed concentrate composition (Table 14) on theperformance of pre-leach. Each test was conducted using the same feedacid (Table 15), with an acid to concentrate contact ratio of 8.4 gramsof P in acid per gram of Ca in the concentrate, at 30° C. for two hours.The results are summarised in Table 16.

TABLE 14 Feed concentrate composition (% w/w or g/t) LRE MRE HRE TRE MgAl P S Ca Fe Th U Test % % g/t % % % % % % % % g/t 24 5.30 0.21 365 5.620.25 1.47 12.8 0.24 29.3 1.24 0.48 355 25 5.19 0.20 356 5.50 0.07 1.3713.0 0.19 29.3 2.11 0.49 395 26 3.71 0.14 227 3.92 0.37 1.01 11.2 0.1331.7 0.67 0.44 217 27 5.50 0.21 387 5.83 0.04 4.18 11.7 0.07 22.4 0.440.57 415 28 5.87 0.22 348 6.19 0.01 0.08 16.0 0.24 34.6 0.25 0.90 373

TABLE 15 Feed acid composition (g/L or mg/L) LRE MRE HRE TRE Mg Al P SCa Fe Th U Test mg/L mg/L mg/t mg/L g/L g/L g/L mg/L g/L g/L mg/L mg/L24 to 28 0.3 — — 0.4 6.7 6.6 207 <10 4.3 6.2 0.4 <0.001

TABLE 16 Dissolution extent (%) Test LRE MRE HRE TRE Mg Al P S Ca Fe ThU 24 36.6 42.9 47.2 37.1 76.5 9.0 83.2 22.8 87.1 11.6 38.0 31.0 25 34.845.4 52.1 35.5 44.3 2.8 83.7 85.8 84.0 1.6 32.7 38.4 26 72.4 74.4 75.172.5 17.0 9.7 87.5 87.3 86.8 11.7 74.1 76.0 27 18.4 27.1 34.8 19.1 46.816.6 63.1 59.3 72.0 3.8 46.3 26.1 28 38.5 43.1 49.9 38.8 41.4 17.0 83.189.1 87.5 0.7 27.7 34.5The Influence of Feed Acid Composition

Five pre-leach tests were conducted to evaluate the influence ofvariability in the feed acid composition (Table 17) on the performanceof pre-leach. Each test was conducted using the same feed concentrate(Table 18), with an acid to concentrate contact ratio of 13 grams ofacid per gram of feed concentrate, at 30° C. for two hours. The resultsare summarised in Table 19.

TABLE 17 Feed acid composition (% w/w or g/t) LRE MRE HRE TRE Mg Al P SCa Fe Th U Test mg/L mg/L mg/L mg/L g/L g/L g/L mg/L g/L g/L mg/L mg/L29 50.3 — — 50.4 3.7 3.6 176 <10 4.7 3.4 <0.1 <0.1 30 47.1 — — 47.3 7.47.2 181 <10 4.9 6.9 1.2 <0.1 31 40.3 — — 40.6 13.3 7.0 174 <10 4.9 6.3<0.1 <0.1 32 30.0 — — 30.2 7.2 10.9 182 510 5.0 6.8 <0.1 <0.1 33 36.0 —— 36.2 7.2 7.3 177 <10 4.9 3.3 <0.1 <0.1

TABLE 18 Feed concentrate composition (% w/w or g/t) LRE MRE HRE TRE MgAl P S Ca Fe Th U Test % % g/t % % % % % % % % g/t 29 to 33 5.57 0.22392 5.90 0.23 1.32 13.1 0.23 30.1 1.16 0.51 377

TABLE 19 Dissolution extent (%) Test LRE MRE HRE TRE Mg Al P S Ca Fe ThU 29 31.2 36.7 46.3 31.7 80.4 0 85.2 90.5 90.1 6.9 31.2 25.7 30 30.135.3 41.9 30.5 81.0 3.3 74.5 82.9 79.4 8.4 31.6 26.2 31 25.2 27.6 32.925.5 79.5 0.3 63.0 72.7 68.1 6.2 26.9 19.9 32 29.4 31.9 37.9 29.7 80.82.0 69.1 77.2 74.0 7.8 32.2 23.5 33 31.6 32.1 38.4 31.8 80.9 1.6 76.384.3 81.2 7.9 33.5 25.0Continuous Two Stage Counter Current Leach

A continuous two-stage pre-leach circuit test was conducted (Test 34).For this test, a thickener was used for first stage solid liquidseparation, while thickening with filtration of thickener underflow wasused for solid liquid separation in the second stage. The first stagefeatured a single tank with 30 minute residence time operated at 40 to45° C., while the second stage contained two 30 minute residence timetanks operated at 30° C. Stage one thickener underflow fed into thefirst stage two leach tank along with phosphoric acid (Table 20). Stagetwo thickener overflow was combined with primary filtrate and spent washsolution, then fed into the stage one leach tank along with damp (9%moisture) concentrate (Table 21). Stage one thickener overflow wascontinuously withdrawn from the system as was washed leach residue cakefrom stage two. For every kilogram of concentrate (dry basis) feedinginto the stage one leach tank, 10.6 kg of phosphoric acid was fed intothe first stage two leach tank. The circuit performance is summarised inTable 22.

TABLE 20 Feed acid composition (g/L or mg/L) LRE MRE HRE Y TRE Al P S CaFe Th U Test g/L mg/L mg/L mg/L g/L g/L g/L g/L g/L g/L g/L g/L 34 — — —— — 5.7 212 2 6.3 7.2 — —

TABLE 21 Feed concentrate composition (% w/w or g/t) LRE MRE HRE Y TREAl P S Ca Fe Th U Test % % g/t g/t % % % % % % % g/t 34 5.6 0.24 413 8355.9 1.5 12.4 0.2 29.6 1.3 0.46 422

TABLE 22 Pre-leach residue composition (% w/w or g/t) LRE MRE HRE Y TREAl P S Ca Fe Th U Test % % g/t g/t % g/t % % % g/t % g/t 34 11.2 0.42663 1280 11.7 3.8 6.5 1.9 12.5 3.0 0.97 852Rare Earth Recovery

Rare earth recovery tests were conducted by heating a measured quantityof pre-leach solution (rare earth recovery feed solution) to boiling ina suitable well agitated baffled vessel fitted with a reflux condenser.The resulting mixture was maintained under a continuous state of boilingfor 120 minutes, then vacuum filtered. The filter cake was then washedthoroughly with DI water to remove entrained solution prior to drying.

Five rare earth recovery tests were conducted to evaluate the influenceof variability in the feed solution composition (Table 23) on theperformance of rare earth recovery. The results are summarised in Tables24 and 25.

TABLE 23 Feed solution composition (% w/w or g/t) LRE MRE HRE Y TRE MgAl P Ca Fe Th U Test g/L mg/L mg/L mg/L g/L g/L g/L g/L g/L g/L mg/Lmg/L 35 2.11 112 23 42 2.28 3.83 3.58 192 36.5 3.44 139 14 36 1.95 10321 38 2.11 7.45 7.16 196 33.2 6.73 138 14 37 1.74 88 18 33 1.88 14.17.20 182 29.5 6.70 114 12 38 1.84 95 20 36 1.99 7.40 10.9 186 31.5 6.90129 12 39 1.91 105 22 38 2.07 7.09 7.00 182 32.8 3.30 141 14

TABLE 24 Precipitate composition (% w/w or g/t) LRE MRE HRE Y TRE Mg AlP Ca Fe Th U Test % % g/t g/t % g/t % % % % % g/t 35 31.2 1.05 703 64532.4 <60 0.13 11.7 4.56 0.34 2.97 24 36 25.6 0.74 499 469 26.4 <60 0.179.16 3.42 0.48 2.44 8.0 37 25.3 0.77 562 532 26.2 <60 0.16 9.21 3.370.70 2.33 8.9 38 24.1 0.66 455 411 24.8 <60 0.19 8.82 3.22 0.56 2.29 1139 25.7 0.75 530 500 26.5 <60 0.19 9.08 3.47 0.27 2.52 8.8

TABLE 25 Precipitation extent (%) Test LRE MRE HRE Y TRE Mg Al P Ca FeTh U 35 80.1 52.1 22.6 7.6 76.9 — 0.18 0.30 0.61 0.49 91.8 0.82 36 71.242.2 16.9 6.4 68.1 — 0.12 0.24 0.52 0.37 80.0 0.30 37 81.1 47.1 17.8 8.077.5 — 0.11 0.25 0.56 0.51 85.4 0.38 38 68.6 35.2 11.5 5.6 65.2 — 0.080.23 0.50 0.39 76.0 0.43 39 73.6 42.6 17.1 6.8 70.1 — 0.14 0.26 0.540.41 83.1 0.33Phosphoric Acid Regeneration

A continuous phosphoric acid regeneration circuit test was conducted(Test 40). The circuit featured a single 55 minute residence timeprecipitation tank (Tank 1) operated at 44° C., followed by a 78 minutestabilisation tank (Tank 2) and batch vacuum filtration, which wassupported by duty standby filter feed tanks and was operated withcounter current washing. Recovery solution was fed into theprecipitation tank along with mixed acid (Table 26). The regenerationperformance is summarised in Tables 27 and 29.

TABLE 26 Feed solution composition (g/L or mg/L) LRE MRE HRE Y TRE Al PS Ca Fe Th U Solution mg/L mg/L mg/L mg/L mg/L g/L g/L g/L g/L g/L mg/Lmg/L Recovery 570 94 40 116 704 4.87 184 0.82 27.0 5.42 26 53 Mixed — —— — — 3.2 84 379 — 3.2 — —

TABLE 27 Precipitate composition (% w/w or g/t) LRE MRE HRE Y TRE Al P SCa Fe Th U Tank g/t g/t g/t g/t g/t g/t % % % g/t g/t g/t 1 1134 85 2452 1244 1852 1.79 19.2 25.4 420 31 2.2 2 1082 86 24 54 1192 1799 1.6819.2 25.3 420 31 1.9

TABLE 28 Regenerated solution composition (g/L or mg/L) LRE MRE HRE YTRE Al P S Ca Fe Th U Tank mg/L mg/L mg/L mg/L mg/L g/L g/L g/L g/L g/Lmg/L mg/L 1 436 83 38 105 558 4.75 19.4 2.22 3.91 5.88 26.5 53.1 2 46186 39 114 586 4.63 19.3 1.66 3.82 5.91 27.2 53.8

TABLE 29 Overall precipitation extent (%) Tank LRE MRE HRE Y TRE Al P SCa Fe Th U 1 21.0 9.5 6.0 4.8 18.5 3.8 0.9 89.8 86.9 0.7 10.8 0.4 2 18.08.5 5.5 4.2 16.0 3.5 0.8 91.5 86.1 0.7 9.5 0.3Acid Bake and Water Leach

Acid bake water leach tests were conducted by contacting a measuredquantity of sulphuric acid with a measured quantity of pre-leach residuewith thorough mixing in a suitable dish. The resulting mixture wasplaced in a furnace and raised to 250° C. over a period of up to 50minutes, then held at 250° C. for a period of 30 minutes, withdrawn fromfurnace and allowed to cool. The cool sulphated material was then addedto a measured quantity of 5° C. DI water, agitated for 10 minutes, thenvacuum filtered. The filter cake was then washed thoroughly with DIwater to remove entrained solution prior to drying.

Six acid bake water leach tests (Tests 41 to 46) were conducted toevaluate the influence of variability in the feed pre-leach residuecomposition (Table 30) on the performance of acid bake water leach. Eachtest was conducted using an acid to residue contact ratio of 1600 kg ofH₂SO₄ per tonne of leach residue, and 2.5 g of DI per gram of pre-leachresidue. The results are summarised in Tables 31 to 33.

TABLE 30 Feed pre-leach residue composition (% w/w or g/t) LRE MRE HRE YTRE Al P S Ca Fe Th U Test % % g/t % % % % % % % % g/t 41 11.2 0.40 6420.13 11.7 4.45 7.16 0.64 12.5 3.64 0.98 814 42 10.1 0.33 510 0.10 10.63.99 6.33 0.08 14.0 6.20 0.99 729 43 4.15 0.15 229 0.05 4.36 3.69 5.670.07 17.0 2.40 0.46 211 44 8.44 0.29 475 0.09 8.86 6.56 8.07 0.05 11.80.80 0.58 577 45 18.5 0.65 891 0.19 19.4 0.36 13.9 0.13 22.2 1.28 3.331250 46 12.5 0.44 706 0.14 13.1 9.66 6.46 0.02 2.29 1.85 1.42 839

TABLE 31 Water leach solution composition (g/L or mg/L) LRE MRE HRE YTRE Al P S Ca Fe Th U Test g/L g/L mg/L mg/L g/L g/L g/L g/L g/L g/L g/Lmg/L 41 34.9 1.24 170 308 36.7 1.79 22.7 104 0.58 3.08 2.79 280 42 28.00.86 108 193 29.2 1.56 18.7 108 0.86 7.35 2.50 219 43 10.0 0.32 34 6510.4 0.53 16.7 98.6 1.30 0.20 1.06 66 44 25.3 0.81 97 183 26.4 0.87 23.392.9 1.46 0.16 1.56 198 45 52.1 1.71 207 372 52.1 0.95 38.7 86.0 0.613.48 4.68 374 46 42.5 1.49 220 382 42.5 0.58 17.9 96.4 1.37 0.27 4.35315

TABLE 32 Water leach residue composition (% w/w or g/t) LRE MRE HRE YTRE Al P S Ca Fe Th U Test % g/t g/t g/t % % % % % % g/t g/t 41 0.26 264118 200 0.32 3.93 0.54 13.8 10.6 2.44 251 16.5 42 0.29 193 83 153 0.344.07 0.36 8.89 12.2 3.63 141 21.6 43 0.18 152 62 116 0.21 3.55 0.23 9.4512.4 2.10 480 12.4 44 0.10 139 62 109 0.13 5.56 0.92 14.3 5.34 0.66 2678.6 45 0.68 897 261 433 0.84 0.35 0.34 21.3 25.9 0.26 4660 24.9 46 0.1273 56 110 0.14 7.47 1.15 14.7 4.15 1.35 609 14.6

TABLE 33 Dissolution extent (%) Test LRE MRE HRE Y TRE Al P S^([1]) CaFe Th U 41 98.0 94.5 84.1 85.0 97.7 1.4 93.9 25.5 1.5 31.7 97.6 98.4 4297.9 95.5 86.3 85.8 97.7 15.5 96.2 14.5 2.2 49.3 98.8 98.0 43 95.5 89.068.1 68.3 94.9 5.4 96.5 19.9 2.8 3.5 89.4 95.3 44 98.5 94.0 80.9 82.098.2 4.1 87.4 36.0 3.8 6.2 94.1 98.4 45 97.4 90.3 79.4 80.7 96.9 56.998.2 33.5 1.0 86.7 83.0 98.6 46 98.8 97.8 89.7 88.5 98.6 1.7 77.4 40.816.8 4.2 94.0 97.9 ^([1])deportment from sulphuric acid to water leachresidue for SRare Earth Sulphate Dissolution

Rare earth sulphate dissolution tests are typically conducted bycontacting a measured quantity of dry rare earth sulphate precipitatewith a measured quantity of DI water with thorough mixing in a suitablewell agitated baffled vessel, at 40° C. for a period of 120 minutes,then vacuum filtered. The filter cake was then washed thoroughly with DIwater to remove entrained solution prior to drying.

Five rare earth sulphate dissolution tests (Tests 47 to 51) wereconducted to evaluate the influence of variability in the feedcomposition (Table 34) on the performance of dissolution. Each test wasconducted using a water to feed solids contact ratio of 13 grams of DIwater per gram of feed solid. Test 47 was operated with a 60 minutedissolution at 22° C., while tests 48 to 51 were operated with a 120minute dissolution at 40° C. The results are summarised in Tables 35 to37.

TABLE 34 Feed rare earth sulphate composition (% w/w or g/t) LRE MRE HREY TRE Al P S Ca Fe Th U Test % % g/t g/t % g/t % % % g/t % g/t 47 35.21.05 833 1363 36.5 9185 3.88 12.2 1.97 6902 2.51 123 48 40.0 1.18 7121050 41.4 <53 0.22 14.7 0.32 560 0.61 1.4 49 43.4 1.28 999 1680 44.9 2120.24 14.9 0.51 769 1.47 3.7 50 41.2 0.97 898 1520 42.4 <53 0.23 14.70.17 490 1.88 2.4 51 40.3 1.21 611 757 41.6 476 0.23 14.1 0.41 629 0.761.6

TABLE 35 Dissolution solution composition (g/L or mg/L) LRE MRE HRE YTRE Al P S Ca Fe Th U Test g/L mg/L mg/L mg/L g/L mg/L mg/L g/L mg/Lmg/L mg/L mg/L 47 11.4 401 43 68 11.9 500 2950 6.28 690 400 4.8 3.3 4834.5 1120 92 78 35.8 190 5 14.3 223 10 103 0.15 49 29.6 1084 94 119 30.920 <3 12.1 293 20 286 0.38 50 34.6 843 91 122 35.6 <10 4 12.8 239 <10345 0.17 51 27.2 875 70 72 28.2 <10 <3 10.1 166 10 110 0.19

TABLE 36 Dissolution residue composition (% w/w or g/t) LRE MRE HRE YTRE Al P S Ca Fe Th U Test % % g/t g/t % g/t % % g/t g/t % g/t 47 35.20.91 471 536 36.2 5716 8.03 7.53 19900 14618 4.97 145 48 36.9 0.86 180117 37.8 1799 10.2 0.35 <72 5735 23.3 3.8 49 22.5 0.53 168 142 23.1 2657.77 8.01 1001 5106 39.1 3.7 50 5.01 0.12 51 41 5.13 <53 8.25 2.82 <715456 51.0 4.1 51 30.8 0.64 105 46 31.4 318 8.65 4.13 786 5664 21.8 3.7

TABLE 37 Dissolution extent (%) Test LRE MRE HRE Y TRE Al P S Ca Fe Th U47 53.5 59.5 73.7 81.7 53.8 71.0 3.5 71.3 53.0 1.3 7.6 45.0 48 97.8 98.299.4 99.7 97.8 — 2.7 99.9 82.2 21.2 6.2 93.2 49 98.7 99.0 99.6 99.8 98.796.9 — 98.7 99.5 32.3 34.1 97.5 50 99.9 99.9 99.9 100 99.9 — 2.0 99.8 —— 73.1 98.3 51 98.2 98.8 99.6 99.9 98.2 98.4 — 99.3 99.6 20.9 32.6 94.5Rare Earth Purification

Rare earth purification tests (Tests 52 and 53) were conducted by dosingmagnesia, to a pH 5 endpoint target, to a measured quantity of impurerare earth sulphate solution at 40° C. in a suitable well agitatedbaffled vessel with online pH measurement. The resulting mixture wasmixed for a period of 30 minutes following magnesia addition to allow itto stabilise, then vacuum filtered. The filter cake was then washedthoroughly with DI water to remove entrained solution prior to drying.

Two rare earth purification tests were conducted to evaluate theinfluence of variability in the source magnesia (Table 38) on theperformance of rare earth purification from a common feed solution(Table 39). The results are summarised in Tables 40 to 42. In both tests(52 and 53), 0.16 kg of MgO was dosed per tonne of feed solution. Therewas 97.3% utilisation of magnesia in Test 52, and a 91.1% utilisation ofmagnesia in Test 53.

TABLE 38 Magnesia composition (% w/w or g/t) LRE MRE HRE TRE Mg Al P SCa Fe Th U Test g/t g/t g/t g/t % % g/t g/t % % g/t g/t 52 28 9.8 3.7 4939.9 0.09   22 80 2.42 0.18 <1 0.15 53 26 8.6 3.8 46 38.3 0.05  <4 400.51 0.06   5 0.09

TABLE 39 Feed solution composition (g/L or mg/L) LRE MRE HRE TRE Mg Al PS Ca Fe Th U Test g/L mg/L mg/L g/L mg/L mg/L mg/L g/L mg/L mg/L mg/Lmg/L 52 & 53 20.2 725 48 21.1 <2 <10 14 7.64 88 <1 113 0.76

TABLE 40 Precipitate composition (% w/w or g/t) LRE MRE HRE TRE Mg Al PS Ca Fe Th U Test % % g/t % % % g/t % % % % g/t 52 30.1 2.38 1533 32.70.61 1.36 882 7.33 0.21 2.06 18.5 17.2 53 24.5 1.74 1030 26.4 2.85 1.831157 6.05 0.10 2.67 22.7 17.2

TABLE 41 Purified rare earth sulphate solution composition (g/L or mg/L)LRE MRE HRE TRE Mg Al P S Ca Fe Th U Test g/L mg/L mg/L g/L mg/L mg/Lmg/L g/L mg/L mg/L mg/L mg/L 52 20.1 704 45 20.9 79 <10 11 8.48 92 <10.56 0.45 53 19.8 722 45 20.7 65 <10 <3 8.29 89 <1 1.74 0.13

TABLE 42 Precipitation extent (%) Test LRE MRE HRE TRE Mg Al P S Ca FeTh U 52 0.44 0.97 0.95 0.46 — — 1.9 0.28 0.67 — 99.5 41.2 53 0.23 0.460.42 0.24 — — 1.6 0.15 0.22 — 98.5 82.9RE Hydroxide PrecipitationThe Influence of Reagent

Rare earth hydroxide precipitation tests (Tests 54 to 56) were conductedby contacting a measured quantity of magnesia or sodium hydroxide to ameasured quantity of purified rare earth sulphate solution at 55° C. ina suitable well agitated baffled vessel. The resulting mixture was mixedfor a period of 30 minutes following each dose of reagent addition toallow it to stabilise, then subsample collected, and vacuum filtered.Subsample filter cake was then washed thoroughly with DI water to removeentrained solution prior to drying. For each test, a range of subsampleswere collected to cover a range of reagent doses. Following the finalprecipitation subsample, a measured quantity of hydrogen peroxide wasadded to the remaining slurry. The resulting mixture was mixed for aperiod of 30 minutes to allow it to stabilise, then vacuum filtered.

Two of the rare earth hydroxide precipitation tests (Test 54 and 55)were conducted to evaluate the influence of variability in the source ofmagnesia (Table 43) on the performance of rare earth hydroxideprecipitation, while the third test (Test 56) evaluated the use ofsodium hydroxide. All three tests were based on a common feed solution(Table 44). Reagent dosing is summarised in Table 45. The results aresummarised in Tables 46 and 47. From the results it can be seen that theuse of magnesia results in significantly increased impurities in theresultant precipitate relative to precipitation using sodium hydroxide.In addition, increased dose rates result in reduced overall deportmentof sulphate to rare earth hydroxide precipitate.

TABLE 43 Magnesia composition (% w/w or g/t) LRE MRE HRE TRE Mg Al P SCa Fe Th U Test g/t g/t g/t g/t % % g/t g/t % % g/t g/t 54 28 9.8 3.7 4939.9 0.09   22 80 2.42 0.18 <1 0.15 55 26 8.6 3.8 46 38.3 0.05  <4 400.51 0.06   5 0.09

TABLE 44 Feed solution composition (g/L or mg/L) LRE MRE HRE TRE Mg Al PS Ca Fe Th U Test g/L mg/L mg/L g/L mg/L mg/L mg/L g/L mg/L mg/L mg/Lmg/L 54 to 56 20.5 760 48 21.4 72 <10 <3 7.49 90 <1 0.73 0.21

TABLE 45 Reagent Addition ^([1]) Sample A B C D E 54 8.6 9.1 9.6 10.2125 55 8.6 9.0 9.6 10.1 126 56 16.9 17.9 19.0 20.1 126 ^([1]) sodiumhydroxide (Test 56) addition, where dosing is in the units kg Magnesiaor NaOH per t of feed. Subsample E corresponds to a period of hydrogenperoxide dosing with reagent addition expressed as a percentage ofstoichiometry.

TABLE 46 Precipitate composition (% w/w or g/t) LRE MRE HRE TRE Mg Al PS Ca Fe Th U Test % % g/t % % g/t g/t % % g/t g/t g/t 54A 51.9 1.52 71353.6 2.80 318 109 6.57 0.17 769 109 1.25 54B 53.9 1.60 744 55.7 2.98 31892 5.73 0.16 839 78 1.48 54C 57.0 1.67 783 58.9 2.46 265 92 4.25 0.21909 82 1.33 54D 56.6 1.65 777 58.4 2.76 265 83 3.45 0.14 909 84 1.34 54E58.6 1.75 826 60.5 2.48 529 92 3.04 0.17 1049 87 1.44 55A 51.4 1.54 73353.1 2.35 212 92 6.77 0.06 489 74 1.16 55B 50.7 1.51 715 52.4 2.36 15987 5.81 0.15 489 75 1.17 55C 50.6 1.50 717 52.3 2.40 159 87 5.17 0.20489 74 1.21 55D 50.1 1.50 710 51.7 2.81 159 83 4.85 0.21 559 73 1.19 55E52.7 1.57 762 54.4 2.53 212 92 4.41 0.20 629 79 1.21 56A 58.8 1.73 84460.8 0.23 <53 83 3.10 0.34 489 86 1.33 56B 63.8 1.89 934 65.9 0.23 <5396 2.60 0.41 489 97 1.48 56C 63.1 1.88 886 65.2 0.22 <53 92 1.60 0.46489 93 1.48 56D 63.9 1.90 907 66.0 0.22 <53 74 1.17 0.44 420 96 1.55 56E67.8 2.04 968 70.1 0.23 <53 87 0.46 0.51 489 97 2.20

TABLE 47 Precipitation extent (%) Test LRE MRE HRE TRE Mg Al P S Ca FeTh U 54A 99.4 99.3 99.2 99.4 — — — 38.4 22.5 — 90.3 97 54B 99.9 99.999.8 99.9 — — — 33.0 20.7 — 90.3 >97 54C 100 99.9 100 100 — — — 23.322.3 — 92.2 >97 54D 99.9 99.9 99.9 99.9 — — — 17.9 15.2 — 93.8 >97 54E99.9 99.9 100 99.9 — — — 14.6 17.4 — 94.1 >97 55A 99.5 100 100 99.5 — —— 36.5 15.4 — 93.5 >97 55B 99.9 99.9 99.9 99.9 — — — 30.1 36.8 —94.9 >97 55C 100 99.8 99.5 99.9 — — — 31.8 48.1 — 90.5 >97 55D 99.9 99.999.9 99.9 — — — 24.9 47.6 — 87.9 >97 55E 99.9 99.9 99.9 99.9 — — — 22.448.0 — 92.8 >97 56A 99.8 99.8 99.7 99.8 — — — 13.5 86.5 — 93.3 >96 56B99.9 99.9 99.9 99.9 — — — 11.3 93.7 — 94.4 >96 56C 99.9 99.8 99.9 99.9 —— — 6.6 >97 — 95.1 >95 56D 99.8 99.8 99.8 99.8 — — — 4.6 >97 — 95.3 >9556E 99.9 99.9 99.9 99.9 — — — 1.7 >97 — 95.8 >95Two Stage Rare Earth Hydroxide Production

A rare earth hydroxide precipitation test (Test 57) was conducted bycontacting a measured quantity sodium hydroxide (97% of stoichiometry)to a measured quantity of purified rare earth sulphate solution (Table48) at 55° C. in a suitable well agitated baffled vessel. The resultingmixture was mixed for a period of 30 minutes to allow it to stabilise,then a measured quantity of hydrogen peroxide (117% of stoichiometry)was added. The resulting mixture was mixed for a period of 30 minutes toallow it to stabilise, then vacuum filtered.

For the second stage, the unwashed cake from the first stage wasrepulped in DI water along with a measured quantity of sodium hydroxide(same mass as first stage). The mixture was agitated for 60 minutes at55° C. then vacuum filtered. The resultant cake was washed.

The results are summarised in Tables 49 to 50.

TABLE 48 Feed solution composition (g/L or mg/L) LRE MRE HRE TRE Mg Al PS Ca Fe Th U Test g/L mg/L mg/L g/L mg/L mg/L mg/L g/L mg/L mg/L mg/Lmg/L 57 20.5 632 39 21.2 80 <10 <3 8.12 283 <10 1.2 0.05

TABLE 49 Precipitate composition (% w/w or g/t) LRE MRE HRE TRE Mg Al PS Ca Fe Th U Stage % % g/t % % g/t g/t % % g/t g/t g/t 1 64.6 1.57 74866.4 0.29 <53 175 2.38 0.33 350 69 2.09 2 66.9 1.69 802 68.8 0.34 <53 870.30 0.46 280 73 2.36

TABLE 50 Precipitation (Stage 1) or dissolution (Stage 2) extent (%)Stage LRE MRE HRE TRE Mg Al P S Ca Fe Th U 1 99.9 99.8 99.9 99.9 97.0 —— 10.0 75.6 — 97.9 >80 2 0 0 0 0 0 0 0 93 0 0 5 21Selective DissolutionThe Influence of Rare Earth Concentration

A series of four two stage rare earth hydroxide dissolution tests (Tests58 to 61) were conducted at 70° C. in a suitable well agitated baffledvessel with online pH measurement. Each test starts by simulating thesecond stage by dosing with a measured portion of 10% w/w hydrochloricacid to a measured portion of rare earth hydroxide cake (Table 51) thathas been repulped in DI water (Test 58) or rare earth chloride solution(Tests 59 to 61), followed by a 30 minute period of stabilisation.Typically, this results in a slurry pH of around 1.2 to 2.2. Each testthen concludes with the second stage by adding a measured quantity ofrare earth hydroxide cake (typically 1.5 times that used to initiate thetest), observing a 30 minute period of stabilisation, then dosing with ameasured portion of 10% w/w hydrochloric acid, followed by a 30 minuteperiod of stabilisation. Typically, this results in a slurry pH ofaround 3 to 4. The test is then concluded with vacuum filtration,followed by cake washing.

Tests 59 to 61 were initiated using rare earth chloride solution fromtest 58, with 2.8, 5.3, and 8.4 g of rare earth chloride solution addedto tests 59 to 61 respectively per gram of rare earth hydroxide consumedby each test. The results are summarised in Tables 52 to 53.

TABLE 51 Feed Solid composition (% w/w or g/t) LRE MRE HRE TRE Mg Al P SCa Fe Th U Test % % g/t % % g/t g/t % % g/t g/t g/t 58 to 61 66.9 1.69802 68.8 0.34 <53 87 0.30 0.46 280 73 2.36

TABLE 52 Final dissolution solution composition (g/L or mg/L) LRE MREHRE Ce Mg Al P S Ca Fe Th U Test g/L g/L mg/L mg/L mg/L mg/L mg/L mg/Lmg/L mg/L mg/L mg/L 58 51.2 2.16 110 <1 482 30 <3 <1 1330 <10 0.14 <0.0159 137 5.14 261 <1 1220 60 <3 <1 3620 <10 0.05 <0.01 60 259 10.0 501 <12180 110 <3 <1 6820 <10 0.04 <0.01 61 366 14.0 704 <1 2970 150 <3 409400 <10 0.03 <0.01

TABLE 53 Overall dissolution extent (%) Test La Ce Pr Nd MRE HRE Y Mg AlCa Th 58 94.8 <0.0021 82.0 78.3 69.1 60.6 67.6 94.2 — 98.7 0.9 59 92.5<0.0015 73.1 67.1 49.0 39.9 39.0 90.6 — 97.3 0 60 94.8 <0.0013 79.2 74.658.6 46.6 45.3 89.9 — 98.5 0 61 95.2 <0.0012 80.4 75.1 57.4 43.4 43.787.6 — 98.5 0Two Stage Selective Dissolution

A two stage rare earth hydroxide dissolution test (Test 62) wasconducted at 70° C. in a suitable well agitated baffled vessel withonline pH measurement. The test was initiated by repulping 60% of therare earth hydroxide cake (Table 54) in de-ionised water followed by thecontrolled dosing of 10% w/w hydrochloric acid to online pH targets ofpH 2 (sample 1), then pH 1 (sample 2). At pH 1, all of the rare earthhydroxide precipitate had been dissolved. The remaining 40% of the rareearth hydroxide cake was then added and the mixture allowed to stabiliseunder agitation for 30 minutes (sample 3). Controlled acid addition wasthen resumed to pH 3 (sample 4), pH 2 (sample 5), and pH 1 (sample 6).This time the rare earth hydroxide cake did not completely dissolve. Thefinal slurry was allowed to agitate for a further 50 minutes prior tocollection of the final sample (sample 7). Following each period of aciddosing and achievement of an on-line pH target, the resultant mixturewas allowed to mix for at least 15 minutes prior to sample collection.Subsamples were vacuum filtered, followed by cake washing.

The results are summarised in Tables 55 to 57. From the results, despitecompletely dissolving all the cerium in rare earth hydroxide by sample2, the addition of additional rare earth hydroxide drove areprecipitation such that by sample 4 (targeting pH 3, achieving pH 3.2after stabilisation) the concentration of cerium in solution was belowdetection limit. This suggests that the cerium dissolved as cerium IV inthis test.

TABLE 54 Feed Solid composition (% w/w or g/t) LRE MRE HRE Y Mg Al P SCa Fe Th U Test % % g/t g/t % g/t g/t % % g/t g/t g/t 62 68.3 1.81 21635290 0.18 159 148 0.32 0.49 420 3.5 1.9

TABLE 55 Subsample dissolution solution composition (g/L or mg/L) La CePr Nd MRE HRE Y Mg Al S Ca Sample g/L mg/L g/L g/L g/L mg/L mg/L mg/Lmg/L g/L mg/L 1 13.8 142 2.58 8.84 1.21 149 364 56 <20 2 978 2 17.432400 3.51 12.1 1.80 228 500 206 <20 242 1206 3 18.2 20 1.83 5.51 0.2815 32 78 <20 <2 1494 4 22.6 <2 4.16 14.3 1.86 201 552 90 <20 10 1592 520.8 20 4.20 14.5 1.96 224 538 80 <20 4 1426 6 26.2 2900 5.51 19.1 2.60303 760 122 20 24 1784 7 31.2 4940 6.44 22.2 3.17 372 900 130 20 20 2140

TABLE 56 Subsample solid composition (% w/w or g/t) La Ce Pr Nd MRE HREY Mg Al S Ca Fe Sample % % % % % g/t g/t % g/t % g/t g/t 1 1.12 63.10.89 3.29 0.55 894 1530 0.18 265 0.61 286 699 2 — — — — — — — — — — — —3 3.14 47.7 3.05 11.5 2.23 3021 7090 0.15 265 0.46 572 560 4 0.92 63.21.03 3.61 0.69 1194 1560 0.11 423 0.60 214 699 5 0.64 65.0 0.68 2.430.46 735 1020 0.10 212 0.61 214 629 6 0.54 66.0 0.54 1.85 0.35 552 7520.10 1006 0.63 214 1189 7 0.61 68.2 0.56 2.01 0.36 567 842 0.10 265 0.64214 909

TABLE 57 Overall dissolution extent (%) Sample La Ce Pr Nd MRE HRE Y MgAl Ca Th 1 97.2 0.6 88.9 88.2 86.0 82.3 86.9 59.0 — 99.0 16.7 2 100 100100 100 100 100 100 100 100 100 100 3 88.0 0.053 43.1 37.6 13.8 5.8 5.439.5 — 97.1 14.3 4 97.6 <0.005 87.0 86.7 81.6 73.6 85.4 56.5 — 99.2 11.35 98.4 0.057 92.0 91.7 88.9 85.0 90.7 59.2 — 99.2 11.9 6 99.4 13.9 97.497.4 96.5 95.3 97.4 81.4 42.3 99.7 41.5 7 99.4 19.6 97.5 97.4 96.7 95.797.3 81.0 71.7 99.7 39.7

A person skilled in the art will understand that there may be variationsand modifications other than those specifically described. It is to beunderstood that the invention includes all such variations andmodifications. The invention also includes all steps, features,compositions and compounds referred to, or indicated in thisspecification, individually or collectively, and any and allcombinations of any two or more of the steps or features.

The claims defining the invention are as follows:
 1. A method ofprocessing a purified rare earth sulfate solution, the method including:a) contacting the purified rare earth sulphate solution with sodiumhydroxide to precipitate rare earths as rare earth hydroxide, includingthe addition of an oxidant to oxidise cerium contained in the rare earthhydroxide precipitate; and b) selectively leaching the rare earthhydroxide precipitate with hydrochloric acid to form a rare earthchloride solution that contains negligible cerium and a residue thatconsists primarily of cerium (IV) hydroxide.
 2. The processing method ofclaim 1, wherein the precipitation occurs in a two-stage counter-currentprocess, including a precipitation stage and refining stage, wherein thepurified rare earth sulphate solution feeds into the precipitation stagewith spent solution from the refining stage, to precipitate rare earthhydroxide containing sulphate.
 3. The processing method of claim 2,wherein the sulphate containing rare earth hydroxide is subsequentlyconverted to clean rare earth hydroxide in the refining stage with theaddition of fresh sodium hydroxide.
 4. The processing method of claim 2,wherein the oxidant is hydrogen peroxide and/or sodium hypochlorite,which is added to the precipitation and/or refining stages followingsodium hydroxide addition.
 5. The processing method of claim 2, whereinboth the precipitation and the refining stage operate at a temperatureof 40 to 80° C., and for a time of about 30 to 60 minutes, with thestoichiometry of sodium hydroxide addition being in the range of 100 to110%, and with 100 to 130% of stoichiometry dosing of oxidant.
 6. Theprocessing method of claim 1, wherein the selective leaching of rareearth hydroxide precipitate is conducted in two stages, each stagehaving multiple tanks and each stage using hydrochloric acid diluted to5 to 15% w/w with leach solution from the first leach stage.
 7. Theprocessing method of claim 6, wherein leach solution from the secondleach stage is used to re-pulp, leach rare earth hydroxide precipitate,and precipitate cerium (IV) dissolved in the second leach stage, priorto the first leach stage.
 8. The processing method of claim 6, whereinthe selective leaching occurs at a temperature of 60 to 80° C., thefirst leach stage is operated to maximise rare earth dissolution whileminimising cerium (IV) dissolution, which is achieved with an endpointpH of about pH 3 to 4, and the second leach stage is operated tominimise the concentration of non-cerium rare earth elements in theresidue.
 9. The processing method of claim 1, wherein barium chloride isadded to the rare earth chloride solution, with sulphuric acid whensulphate levels are low, to remove radium via co-precipitation withbarium sulphate to form a purified rare earth chloride solution.
 10. Themethod of claim 8, wherein the purified rare earth chloride solution isconcentrated by evaporation.